Abstract
Amorphous alloys are a new type of metallic materials with many potential applications due to the combianation of a set of unqique properties, including high strength and hardness, good elasticity, excellent wear- and corrosion-resistance. However,amorphous alloys suffer from the limited product size and poor room-temperature plasticity due to the disordering of atomic structure. Fabrication of amorphous coating is thought to be an allernate route to overcome the shortcomings of bulk amorphous alloys.
The aim of this dissertation is therefore to develop high performance amorphous coatings based on the Fe-based amorphous system by thermal spray technique. The preparation, microstructure and properties, including corrosion resistance, friction/wear properties andwetting behavior, of Fe-based amorphous coatings have been systematically investigated by means of X-ray diffraction (XRD), differential scanning calorimetry (DSC) and differential thermal analysis (DTA), optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), nanoindentation,micro-hardness measurement, friction/wear testing system and electrochemical workstation.
Fe48Cr15Mo14C15B6Y2 (nominal compostion) amorhous coatings was successfully fabricated on mild steel substrate by the high velocity oxy-fuel thermal spraying (HVOF) process. The effect of the size of feedstock powders on the microstructure and corrosion resistance of the coatings was investigated. The results showed that the coatings are highly densed with the porosity of less than 1%, and the coatings are nearly fully amorphous except a few nano-oxdies embedding in the intersplat regions. All coatings exhibit good corrosion resistance in 3.5 % NaCl solution with wide spontaneous passive rejion and rather low passive current density. However, the particle size of the feedstock powders had significant influence on the microstrture and the corrosion proterties of the resultant coatings. The coatings sprayed with the finest powders show the most compact structure;while the coating with the coarser powders exhibits a better corrosion resistance. It was also found that the corrosion resistance of the coatings is closely related to the wetting behavior which is affected by the oxygen content and the roughness of coatings. The coatings with hydrophobicity exhibit a better corrosion resistance.
To understand the corrosion mechasim of the amorphous coating, the pitting initiation of the coating in a 6M NaCl solution was studied in detail via high resolution transmission electron microscopy (TEM) coupled with nanobeam energy-dispersive X-ray spectroscopy (EDX). It was found that pitting was always initiated in a narrow region about 100 nm wide near the intersplat regions, but not exactly at the regions as have been expected.
Nanobeam EDX indicated that a Cr-depleted zone exists near the intersplat due to the oxidation effect. More interestingly, pitting was found to occur only on one side although Cr depletion is equal on the two sides of the intersplat. This can be well explained in terms of the galvanic effect between the Cr-depleted zone and the Cr-rich intersplat regions.
The wear behavior of the amorphous coatings was studied under dry sliding conditions in a ball-on-plate mode using alumina ball as the counterpart. It was found that the friction coefficient and the wear rate of the coating are around 0.3-0.4, and (3~19) ×10-5mm3N-1m-1, respectively. Compared with traditional steels and other wear-resistant coatings, such as hard Cr and Al2O3 coatings, the Fe-based amorphous coating shows higher wear resistance. The wear rate is independent of the applied load, but increases linearly with the increase of sliding speed. The wear mechanism of the Fe-based amorphous coating was found to be dominated by oxidative wear due to relatively high flash temperature and large oxygen affinity of alloy elements. Further analysis revealed that the oxidation process is governed by inward diffusion of oxygen. In addition, the delamination wear also occurred at the intersplat regions during wear process.
By carefully designing the spraying parameters and the powders sizes, a variety of Fe-based amorphous metallic coatings with different surface topographies and roughness could be fabricated via thermal spraying. The results showed that the wetiing behavior of the coating could be altered by surface rougness, the coating behaves hydrophilic when the roughness is less than 5μm，while becomes hydrophobic when the roughness is larger than 9μm. The largest static water contact angle can be reach as high as 130°~140° in the study.
Further modification with low surface energy treatment, i.e, supptering a nanostructured Au layer + splating a dodecanethiol film, results in the realization of superhydrophobicity of the coating, which shows the contact angle greater than 150° and the sliding angle less than 16° (for a 8 μL water droplet). The superhydrophobic coating exhibits excellent self-cleaning effect. Finally, the roughness effect on the wetting behavior of the amorphous coatings was discussed in terms of Wenzel and Cassie-Baxter theory.
Keywords: Fe-based amorphous coating; powder size effect; oxidation effect; corrosion resistance; pitting mechanism; wear properties; wetting behavior; hydrophobicity and superhydrophobicity.
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